In order to implement a broadband multimedia wireless service system with a reliable high speed and large capacity, OFDM transmission methods for transmitting signals with high data rates in millimeter wave bandwidths from several to several tens of GHz have been used.
The OFDM method of transmitting a data row on a subcarrier at a lower data transmission rate is the most adequate modulation method for high-rate data transmission at present. However, in a wireless communication channel environment, radio signals are reflected off objects, such as walls, buildings, mountains, etc., causing multipath. A multipath radio channel may cause delay spread. Further, it may cause inter symbol interference (ISI) when the length of the delay spread is greater than a time taken for transmitting the next symbol. The multipath delay spread causes frequency-selective fading in the frequency domain, and an equalizer is used to eliminate ISI components when the system uses a single-carrier. However, the equalizer becomes more complex as data transmission speed increases. Therefore, data are processed in parallel by transmitting a high rate data stream in parallel on a plurality of subcarriers such that the high-rate data stream is split into a number of lower rate streams, which are then simultaneously transmitted on a number of subcarriers in the OFDM system. As described, the ISI is reduced when using low-rate parallel carriers because a symbol interval becomes longer, and the ISI can be almost completely eliminated by using a guard interval. In addition, a further merit of the OFDM system is that the OFDM system is strong against frequency selective fading because it uses multiple carriers.
FIG. 1 shows locations of pilot subcarriers and data subcarriers within each symbol when a scattered pilot is distributed throughout the symbols in an OFDM system, wherein the scattered pilot changes locations from symbol to symbol.
As shown in FIG. 1, each of the OFDM symbols includes a null subcarrier, a pilot subcarrier, and a data subcarrier. The null subcarrier composes a guard interval together with a zero indexed DC, and the pilot subcarrier is used for channel estimation. The data subcarrier fills the remainder of the OFDM symbol.
In the OFDM system, each pilot subcarrier is spaced by 9 subcarriers within a symbol, and then spaced by 3 subcarriers in the next symbol. Thus, a predetermined number of data subcarriers are allocated to the respective symbols. However, a location of the data subcarriers in the symbol may vary depending on a symbol number as shown in FIG. 1.
For example, data subcarrier 0 is mapped to subcarrier 1 in every symbol. However, data subcarrier 1 is mapped to subcarrier 2 in the first and second symbols, and mapped to subcarrier 3 in the third symbol. Therefore, a mapping logic is required to locate data subcarriers on actual subcarriers within a symbol. Herein, an index formed only by the data subcarriers is called a logical index, and the substantial subcarrier location within the symbol is called a physical index.
A simple method for mapping the logical index to the physical index using a Read Only Memory (ROM) table is shown in FIG. 2.
FIG. 2 shows the arrangement of data, pilot, and null subcarriers, with the existence of scattered pilots, in a conventional OFDM system.
As shown in FIG. 2, a result of a fast Fourier transform (FFT) processing unit 10 of a demodulator is classified as a null subcarrier, a data subcarrier, and a pilot subcarrier. The null subcarrier is discarded, and the data subcarrier and the pilot subcarrier are stored in a data buffer 22 and a pilot buffer 24 in an FFT buffer 20, respectively. At this point, a subcarrier mapping ROM table 30 determines whether to discard a subcarrier or to store it in either the data buffer 22 or the pilot buffer 24 according to a physical index output from the FFT processing unit 10.
Thus, the subcarrier mapping ROM table 30 stores information on a subcarrier corresponding to a physical index output from the FFT processing unit 10. The information contains a type and an address of the subcarrier.
The channel estimator 40 estimates channel characteristics of the corresponding subcarrier and its adjacent subcarrier using pilot subcarriers stored in the pilot buffer 24, and the equalizer 50 equalizes data subcarriers with reference to the channel characteristics. The QAM demodulator 60 outputs data to be transmitted to a channel decoder. Then the channel decoder demodulates desired data by performing channel decoding.
However, the size of the subcarrier mapping ROM table 30 becomes increased when locations of the pilot subcarriers change from symbol to symbol as shown in FIG. 1. That is, when scattered pilots are included in the symbols, the number of pilot subcarriers included in the symbols increases and accordingly, the size of the FFT processing unit 10 increases. Thus, it is inefficient to use the subcarrier mapping ROM table 30.
The above information disclosed in this Background of the Invention section is only for enhancement of understanding of the background of the invention and therefore, it should not be understood that all the above information forms the prior art that is already known in this country to a person of ordinary skill in the art.